[PATCH net 1/4] net/udp_gso: Allow TX timestamp with UDP GSO

[Fred Klassen <fklassen@appneta.com>]

Fixes an issue where TX Timestamps are not arriving on the error queue
when UDP_SEGMENT CMSG type is combined with CMSG type SO_TIMESTAMPING.
This can be illustrated with an updated updgso_bench_tx program which
includes the '-T' option to test for this condition.

    ./udpgso_bench_tx -4ucTPv -S 1472 -l2 -D 172.16.120.18
    poll timeout
    udp tx:      0 MB/s        1 calls/s      1 msg/s

The "poll timeout" message above indicates that TX timestamp never
arrived.

It also appears that other TX CMSG types cause similar issues, for
example trying to set SOL_IP/IP_TOS.

    ./udpgso_bench_tx -4ucPv -S 1472 -q 182 -l2 -D 172.16.120.18
    poll timeout
    udp tx:      0 MB/s        1 calls/s      1 msg/s

This patch preserves tx_flags for the first UDP GSO segment. This
mirrors the stack's behaviour for IPv4 fragments.

Fixes: ee80d1ebe5ba ("udp: add udp gso")
Signed-off-by: Fred Klassen <fklassen@appneta.com>
---
 net/ipv4/udp_offload.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 065334b41d57..33de347695ae 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -228,6 +228,10 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	seg = segs;
 	uh = udp_hdr(seg);
 
+	/* preserve TX timestamp and zero-copy info for first segment */
+	skb_shinfo(seg)->tskey = skb_shinfo(gso_skb)->tskey;
+	skb_shinfo(seg)->tx_flags = skb_shinfo(gso_skb)->tx_flags;
+
 	/* compute checksum adjustment based on old length versus new */
 	newlen = htons(sizeof(*uh) + mss);
 	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
-- 
2.11.0